Render slows down in the last couple minutes
Everything is just simple.

i5-4670k heve 8 logical cores wich one by one render slower that one GPU.



Can see, how GPU overtaking CPU cores. And then they remain, but the GPU is not involved in process. There is also an inaccuracy in the calculation of time. 

It would be better if the CPU represented only one tile. I have posted about it on developer.blender.org. 

You need to render only by GPU and it will become faster then with CPU.